Synthetic pyrethroid esters, similar in structure to naturally occurring pyrethrin, are well known as insecticides of high stability and low mammalian toxicity. These synthetic esters are superior to the pyrethrins found in nature in a number of ways. First, the naturally occurring pyrethrins are subject to very fast degradation and their insecticidal activity is neutralized by air and light. Second, the naturally occurring compounds are not available in great abundance and are costly to extract from their natural state. The synthesized variations of these compounds, on the other hand, have a higher stability, and yet are sufficiently degradable that they do not present environmental problems. They are also resistant to light induced oxidation. In addition, pyrethroids have a low toxicity for mammals and humans, relative to other insecticides, while exhibiting high insecticidal activity to a wide variety of insects.
One of the methods of preparation of these synthetic pyrethroids is disclosed in P. E. Burt, M. Elliott, A. W. Farnham, N. F. Janes, P. H. Needham, and D. A. Pullman, Pesticide Science 5, 791-799 (1974). According to this method, ethyldiazoacetate is reacted with 1,1-dichloro-4-methylpenta-1,3-diene to form ethyl(.+-.)-cis, trans-2,2-dimethyl-3-(2,2-dichlorovinyl)-cyclopropanecarboxylate, which is then converted to the carboxylic acid. The latter was subsequently converted to the acid chloride, then reacted with 3-phenoxybenzyl alcohol in a Schotten-Baumann reaction, to produce 3-phenoxybenzyl 2,2-dimethyl-3-(2,2-dichlorovinyl)-cyclopropanecarboxylate, a well known insecticidally active pyrethroid ester. The above-mentioned diene can be prepared by the reaction of an appropriate sulfone with sodium hydroxide in a Ramberg-Backland type rearrangement. See L. Ramberg and B. Backland, Arkiv. Kemi. Mineral. Geol., 13A, No. 27 (1940); also Bordwell and Cooper, J. Am. Chem. Soc., 73, 5187-5190 (1951). The process involves a large number of steps, including those for the preparation of the sulfone, and requires the use of costly reagents.
The diene has also been prepared from chloral and isobutylene, Farkas, Kourim, and Sorm, Collection Czechoslov. Chem. Commun., 24, 2230-2236 (1959), in a four-step process involving a costly zinc elimination.
A simpler process involves the addition of carbon tetrachloride to 3-methyl-1-butene to form 1,1,1,3-tetrachloro-4-methylpentane, followed by a liquid phase dehydrochlorination to form 1,1-dichloro-4-methyl-1,3-pentadiene. A variety of materials are known to catalyze this or similar liquid phase dehydrochlorinations, notably BF.sub.3 and FeCl.sub.3, see Topchiev, Bogomolova, and Gol'dfarb, Doklady Akad. Nauk S.S.S.R., 107, 420-3 (1956), and Belgian Pat. No. 621,439. These known catalytic processes suffer from low yields due to polymerization of the product.
The object of this invention is to provide a novel process for the dehydrohalogenation of a 1,1,1,3-tetrahalo-4-methylpentane which entails polymerization to much less an extent than known processes, and thus produces higher yields of the desired product.